It is not an Outlook problem. It is not an error. The other system is moderating the emails that come in. Call the company or email either postmaster@thatcompany or abuse@thatcompany. Ask them to whitelist your domain.

Some companies reject all emails unless you have first registered with them to allow email functionality.

We have two companies (our support companies) that I had this error with, emails were rejected, upon registering on their website and submitting an online ticket I was also registered to be able to send support via email.